A product is set on the upward horizontal support surface in the screwing apparatus. The driver bits are driven to move downward in the vertical direction toward the product. Screws are held at the tip ends of the driver bits, respectively. The screws are received in the screw bores of the product, respectively. Moving further downward, the driver bits are driven to rotate around the axes of the driver bits, respectively. The screws are in this manner screwed into the screw bores, respectively.
A cover is screwed on an enclosure body to assemble a hard disk drive, for example. Screws, six of those, are utilized for screwing the cover on the enclosure body, for example. The cover is placed on the enclosure body to close the opening of the enclosure body. An elastic packing is sandwiched between the enclosure body and the cover. The screwing apparatus effects the screwing operation simultaneously on all the screws.
The screwing apparatus tends to suffer from differences between the levels of the driver bits during the screwing operation. The differences in the levels cause the cover to suffer from deviation from the horizontal attitude. This results in differences between the screwed amounts of the screws. A sufficient sealing effect thus cannot be obtained between the cover and the enclosure body.